This invention relates to an apparatus and method for calibrating a flow meter utilizing a prover.
In the use of flow meters to measure the quantity of fluid flowing in a conduit, it is frequently desirable to determine the accuracy of the meter in detecting the volume of fluid flowing through the meter. A process for determining meter accuracy is commonly referred to as meter calibration. A meter calibration process typically utilizes a device known as a prover.
As used herein, the term prover refers to any of a number of devices which include a conduit containing a solid member, such as a ball or piston, movably positioned therein, and two detection points along the conduit having suitable means, such as actuating switches, associated therewith for detecting the solid member as it moves past each detection point.
In calibrating a volumetric flow meter, a stream of fluid is passed simultaneously through a prover and the flow meter. Liquid flowing through the prover conduit moves the solid member from one detection point to the other detection point. The output of the flow meter is monitored in the time interval between actuating times for the detection points. This output is correlated with the known volume of the prover conduit between the detection points to yield an appropriate calibration factor.
Some particular problems arise when calibrating a liquid flow meter. More particularly, it is desirable that such a liquid calibration system be liquid-full at all times during the calibration test in view of the fact that a mixed phase of vapor and liquid causes the meter and prover to give erratic and unpredictable readings. Most prior calibration systems have utilized a large liquid supply tank maintained at atmospheric pressure for helping maintain a single liquid phase condition in the calibration system, and an associated pump for pumping liquid from the tank for subsequent flow through the meter and prover. Typically, the suction side of the pump is connected to the supply tank so as to be maintained at atmospheric pressure, and the discharge side of the pump discharges to the meter and prover which are maintained at a higher pressure. The pressure differential across the pump is usually substantial (i.e., 200 psi or more). Since the power requirements for the pump are based on the pressure differential across the pump, the large pressure differential usually requires a pump with an undesirably high horsepower. Generally, an increase in pump power means a resultant increase in heat introduced to the system which contributes to temperature instability of the system and consequent unreliable calibration results. Furthermore, continual contact of ambient air with the liquid in the supply tank causes aeration of the liquid and a two-phase condition which leads to erroneous results as discussed above.